1. Field
The present disclosure relates to techniques for communicating optical signals. More specifically, the present disclosure relates to wavelength-locking a ring-resonator filter without monitoring an input optical signal to the ring-resonator filter.
2. Related Art
Silicon photonics is a promising technology that can provide large communication bandwidth, low latency and low power consumption for inter-chip and intra-chip connections. In the last few years, significant progress has been made in developing low-cost components for use in inter-chip and intra-chip silicon-photonic connections, including: high-bandwidth efficient silicon modulators, low-loss optical waveguides, wavelength-division-multiplexing (WDM) components, and high-speed CMOS optical-waveguide photo-detectors. However, a suitable WDM multiplexer/demultiplexer remains a challenge, the lack of which poses an obstacle to implementing WDM silicon-photonic links.
One approach to implementing a WDM multiplexer/demultiplexer is to use silicon ring resonators as add/drop filters (which are sometimes referred to as ‘ring-resonator filters’). By cascading multiple ring-resonator filters, a multi-channel multiplexer/demultiplexer can be fabricated with desirable optical performance for area-sensitive applications. However, one well-known challenge for practical application of a ring-resonator multiplexer/demultiplexer is the uncertainty in the channel spacing and center wavelength that results from variations in the index of refraction of the effective silicon optical waveguide due to fabrication tolerances and ambient temperature change. Wavelength tuning is typically used to align and lock the ring-resonator filters with the optical signal carrier wavelengths.
There are both direct and indirect techniques for aligning and locking a ring-resonator filter to the carrier wavelength of an input optical signal. For example, temperature sensing and control can be used to indirectly control the resonance of the ring resonator filter. However, it can be difficult to accurately sense the ring-resonator temperature. Consequently, temperature control with sub-Kelvin accuracy is often difficult and typically results in significant power consumption.
For high fidelity, direct control is often used to control the resonance wavelength of the ring-resonator filter. As shown in FIG. 1, which presents a drawing of an existing filter, direct control of the resonance wavelength of the ring-resonator add/drop filter typically requires an optical tap to measure the output signal power of the ring-resonator filter by converting the tapped optical signal into an electrical signal. Then, an electrical controller (or control logic) analyzes the tapped optical signal and drives a thermal tuner (or heater) integrated with the ring-resonator filter to maximize the output of the ring-resonator filter. This control technique is based on the assumption that the power of the input optical signal is constant. In practice, the power of the input optical signal fluctuates. In order to be independent of these input-power variations, an input-power monitor is usually used so that the electrical controller can compare both the input and output power to align the resonant wavelength of the ring-resonator filter with the optical signal carrier wavelength.
However in a multi-channel multiplexer/demultiplexer, the input-power monitor sees the total power of multiple wavelength channels. Consequently, the electrical controller cannot determine the exact power fluctuation of any given channel, which makes it more difficult for the electrical controller to tune the ring-resonator filters.
Hence, what is needed is a ring-resonator filter without the above-described problems.